[clang] [Clang][counted_by] Don't treat a __bdos argument as an array if it isn't (PR #125298)

[Nathan Chancellor via cfe-commits]

nathanchance wrote:

This tentatively looks good to me. Do you need a reduced test case for this? `cvise` gave me:

```c
typedef struct {
  char __padding[0];
} spinlock_t;
struct {
  int priv_len;
  spinlock_t addr_list_lock;
  char *dev_addr;
  char priv[] __attribute__((__counted_by__(priv_len)));
} falcon_reconfigure_xmac_core_efx;
void falcon_reconfigure_xmac_core() {
  long __q_size_field = __builtin_dynamic_object_size(
      &falcon_reconfigure_xmac_core_efx.dev_addr[4], 1);
  _Bool __ret_do_once = __q_size_field, __ret_cond = __ret_do_once;
  static _Bool __already_done;
  __ret_cond &&__already_done;
}
```

https://github.com/llvm/llvm-project/pull/125298